Fatigue crack initiation and propagation of 100Cr6 steel under torsional loading in very high cycle regime

Hongqian Xue,Tao Gao,Zhidan Sun,Xianjie Zhang

doi:10.1051/matecconf/201816520003

Abstract

Cyclic torsional fatigue properties of a high strength steel 100Cr6 are investigated using an ultrasonic torsional fatigue testing machine, and the results are compared with those obtained with fatigue tests under axial loading. Fatigue crack initiation and growth under torsion loading are observed in the very high cycle regime. Results show that fatigue cracks initiated from specimen surface as well as subsurface inclusions under torsion loading. However, subsurface MnS inclusions play a dominant role in crack initiation under torsion loading in the very high cycle regime. The initiation and early propagation of fatigue cracks are mostly controlled by the direction of the maximum shear stress. For surface crack initiation, cracks initiated in parallel to the longitudinal direction of the specimens. Once the shear crack propagated to a crack length of about 20-30 μm, crack branched to the angle close to the direction perpendicular to the remote maximum principal stresses. As for the subsurface fatigue crack initiation, the cracks parallel to the longitudinal direction of the specimens could not be observed, and crack propagation followed a spiral shape on a plane with an orientation of 45° with respect to the loading direction, which corresponds to the maximum principal stress plane.

Highlights

Structural components subjected to high frequency vibrations, such as those used in rotary parts in engines, are usually required to be designed using a lifetime failure-free criterion or an endurance limit for a very large number of cycles
Fatigue data obtained under uniaxial loading are not sufficient to assess very high cycle fatigue limit of mechanical components which are subjected to torsion loading over a wide range of cyclic frequency
Hollow circle and solid circle symbols are used for fatigue crack initiation at surface or subsurface respectively

Summary

Introduction

Structural components subjected to high frequency vibrations, such as those used in rotary parts in engines, are usually required to be designed using a lifetime failure-free criterion or an endurance limit for a very large number of cycles. In the rotary components such as transmission shafts, bearing structural components, fatigue failure occurs under the effect of combined bending and torsional loadings often at very high frequency. It is necessary to investigate the torsion fatigue performance of materials in very high cycle regime in order to appropriately design mechanical parts with respect to the life in service. The components are subjected to low amplitude, high frequency cyclic loading due to for example vehicle vibrations, and the number of cycles to failure is in the very high cycle regime.

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Results

Conclusion